This invention relates generally to the fastener arts and more particularly to a fastener drive system. Still more particularly, the invention relates to a fastener drive system for applications where it is desirable that the fastener be maintained on the end of the drive tool in a co-axial, wobble free orientation, such as in the attachment of a medical prosthesis to bone and other more common type usages.
Medical prosthetic arts have advanced substantially in both design and the materials used creating in prosthetics. Typically, a combination of mechanical fasteners and adhesives are used to secure a prosthesis to surrounding bone. A high degree of precision is required in order to securely retain the prosthesis in a proper working position. When the prosthesis is fitted to surrounding bone it is preferred to make the prosthesis fit correctly the first time to avoid having to make adjustments. As such, it is advantageous to drive fasteners used to secure the prosthesis correctly on the first attempt to provide secure attachment and to avoid unnecessary mounting and remounting of the prosthesis.
Surgeons performing prosthetic installation operations work under most difficult operating conditions. Not only must the surgeon focus his attention on the operating site and any complications which may arise, the operating area is not usually freely accessible creating difficulty in fitting and positioning the prosthesis. Therefore, while attaching the prosthesis the surgeon requires one hand to expose and hold the prosthesis in position leaving only one hand free to drive fasteners. Clearly, fasteners and fastener drive systems used in securing the prosthesis to bone require single-handed operation. It is necessary that the fastener used in such an operation be retained by the driver so that the surgeon can position the fastener and drive it into secure engagement using one hand.
In retaining a fastener on a driver, an easily releasable securely retaining friction fit is preferred. Prior art fastener retaining systems have employed magnetic retainers which although capable of retaining a fastener on the end of the drive tool gives rise to other problems. Primarily, the fastener is not retained securely enough such that when a side force contacts the fastener it comes loose from the driver.
Other prior art fastener retaining systems utilized retaining mechanisms which held a fastener in place through an arrangement of arms or fingers. These retaining mechanisms, however, could interfere with the operation and require additional space within the operation sight for releasing the fastener. Further, the mechanical retaining mechanisms had problems in suppressing fastener wobble as the fastener was driven. The type of engagement between the fastener and the driver is very important since prior art fastener drive systems are clearly inferior for this extremely critical high precision medical application. For example, many prior art systems have problems with "camout", which forces the driver out of the fastener recess, potentially reaming the fastener recess and damaging the driver and operating site. Camout occurs when drive torque is applied to the inclined walls of typical prior art fasteners such as cruciform or phillips type drive recesses. While, in certain situations camout can be overcome by increasing the end load on the driver to more securely force it into the recess, additional end load can create damage in medical procedures. For example, when applying additional end load, should the driver camout of the fastener, the driver may slip into the operating sight and gore tissue, damage bone or damage the prosthesis. Clearly for medical applications, a better alternative must be provided.
As an additional requirement for a fastener in a prosthesis installation application, the fastener must be correctly driven on the first attempt. Correctly driving the fastener on the first attempt provides secure retention of the prosthesis to the bone. The fastener must be able to hold the prosthesis to the bone for the life of the patient and therefore loose fasteners cannot be tolerated. Clearly, if the fastener has to be removed and reinserted for proper seating or because the fastener wobbled while being driven and seated at an angle, an eccentric hole would be created and potentially result in a loose fitting prosthesis.
Prior art mechanisms which securely retain the fastener to the driver have problems with the fastener wobbling about the central axis extending through the fastener and the driver. Typically, the fastener has a recess formed on a top surface of the head and the driver has a cooperatively mating male protrusion which is formed to engage the recess in the fastener. An example of such a fastener and driver combination is the standard hexalobular TORX fastener and corresponding driver. The standard TORX fastener employs a driver bit which, in cross section, has six equidimensioned and equispaced curved lobes which engage in corresponding cross-sectional shaped recesses in the head of the fastener. The sides of the standard TORX fastener are generally parallel to the central axis. While the standard TORX fastener is retainable on the corresponding driver, it has a degree of wobble and is not sufficiently securely retained on the driver for medical applications.
Prior art prosthesis fasteners have attempted to overcome both the problem of retention on the driver to permit a single-handed operation and wobble prevention. One prior art device attempts to overcome these problems by forming the driver bit in a hexalobular form with a spiral twist to the lobes relative to the central axis of the bit. This fastener, while reducing wobble and securely retaining the fastener on the driver has the problem in that the fastener is retained too securely. While the secure retention of the fastener helps in positioning the fastener in the operating sight, the spiral twist makes it difficult to disengage the driver from the fastener. Further, since the spiral in the drive recess follows the thread spiral, the fastener may actually be loosened upon disengaging the driver.
Additionally, the degree of retaining force between the fastener and the driver bit of this prior art system results in shortening the driver life. It is important that the driver is not the critical component in the driver system such that even after some wear causing use the driver can be used to satisfactorily complete an operation should other drive bits be unavailable or damaged. As such, it is preferred that the fastener be prevented from wobbling even after the driver bit has developed a degree of wear.
The present invention, as will be detailed more fully hereinafter, overcomes the above-described problems. More specifically, the present invention provides a fastener drive system which securely retains a fastener on a driver bit, prevents wobbling of the fastener during driving and permits the driver to be easily removed from the fastener once the fastener is installed or seated.